Oxygen Binding to Myoglobin and Hemoglobin

Myoglobin can bind only one molecule of O₂, because it contains only one heme group. In contrast, hemoglobin can bind four molecules of O₂, one at each of its four heme groups. The degree of saturation (Y) of these oxygenbinding sites on all myoglobin or hemoglobin molecules can vary between zero (all sites are empty) and 100 % (all sites are full), as shown in Figure 1. [Note: Pulse oximetry is a noninvasive, indirect method of measuring the oxygen saturation of arterial blood based on differences in light absorption by oxyhemoglobin and deoxyhemoglobin.]

Figure 1: Oxygen-dissociation curves for myoglobin and hemoglobin (Hb).

1. Oxygen-dissociation curve: A plot of Y measured at different partial pressures of oxygen (p_O2) is called the oxygen-dissociation curve. [Note: p_O2 may also be represented as P_O2.] The curves for myoglobin and hemoglobin show important differences (see Fig. 1). This graph illustrates that myoglobin has a higher oxygen affinity at all p_O2 values than does hemoglobin. The partial pressure of oxygen needed to achieve half saturation of the binding sites (P₅₀) is ~1 mm Hg for myoglobin and 26 mm Hg for hemoglobin. The higher the oxygen affinity (that is, the more tightly _O2 binds), the lower the P₅₀.
a. Myoglobin: The oxygen-dissociation curve for myoglobin has a hyperbolic shape (see Fig. 1 ). This reflects the fact that myoglobin reversibly binds a single molecule of O2. Thus, oxygenated (Mb_O2) and deoxygenated (Mb) myoglobin exist in a simple equilibrium:

The equilibrium is shifted to the right or to the left as O₂ is added to or removed from the system. [Note: Myoglobin is designed to bind O₂ released by hemoglobin at the low pO2 found in muscle. Myoglobin, in turn, releases O₂ within the muscle cell in response to oxygen demand.]
b. Hemoglobin: The oxygen-dissociation curve for hemoglobin is sigmoidal in shape (see Fig. 3.6), indicating that the subunits cooperate in binding O₂. Cooperative binding of O₂ by the four subunits of hemoglobin means that the binding of an oxygen molecule at one subunit increases the oxygen affinity of the remaining subunits in the same hemoglobin tetramer (Fig. 2). Although it is more difficult for the first oxygen molecule to bind to hemoglobin, the subsequent binding of oxygen molecules occurs with high affinity, as shown by the steep upward curve in the region near 20–30 mm Hg (see Fig. 1).

Figure 2: Hemoglobin (Hb) binds successive molecules of oxygen (O₂) with increasing affinity.